The invention relates to whitened polyacrylonitriles and acrylonitrile copolymers, especially as aqueous dispersions, to coating slips comprising them, and to their use for coating a wide variety of substrates.
In addition to the customary white pigments, especially china clay and calcium carbonate, the coating slips generally include anionic whiteners for the purpose of optical brightening, examples being those of the bis-triazinyl-aminostilbene-disulfonic acid type. These whiteners, however, lead only to highly unsatisfactory white effects and to very low grayness limits (=whitener concentration at which further addition of whitener produces no increase, or even a decrease, in whiteness). Furthermore, the whitener types referred to possess inadequate lightfastness in the coating slips.
DE-A-195 00 195 describes how whitened, finely divided polymer materials affixed with brighteners suitable for the polymer material can be used as powders or as dispersions for whitening paper coating slips based on synthetic binders. This method is still in need of improvement, however, in terms of the rheological properties when the whitened polymer powder is used, or in terms of the solids contents of their dispersions.
The preparation of whitened coatings on various substrates, especially on coated papers and cards with high whiteness, a high grayness limit and good lightfastness, is a difficult problem which has not been solved satisfactorily to date.
It has now been found, surprisingly, that when specific whitened polymers are used in coating slips, they give the coatings produced therewith on a wide variety of substrates very high whitenesses, high grayness limits, high lightfastnesses, and high bleedfastnesses.
The invention therefore provides polyacrylonitriles or acrylonitrile copolymers, referred to below simply as polymers, which are whitened by means of optical brighteners and are obtainable by emulsion polymerization of acrylonitrile with or without comonomers, in the presence or absence of graftable substrates, with the addition of an optical brightener before, during or after the emulsion polymerization.
In one preferred embodiment the polymers of the invention are in the form of their aqueous dispersions.
The aqueous polymer dispersion contains preferably:
a) 1-65% by weight, in particular 10-55% by weight, of polymer of the invention, based on the dispersion, and
b) 1-15% by weight, in particular 5-10% by weight, of surface-active substances, based on polymer of a).
The aqueous polymer dispersion may additionally include customary additives such as preservatives, preferably from 0 to 15% by weight, based on the polymer of component a).
Suitable surface-active substances are preferably anionic and/or nonionic surfactants.
Preferred surface-active substances are the graftable substrates of types I to III below, preferably those of types I and II, and, in particular, those of type I.
Examples of suitable preservatives are methyl- and chloromethyl-isothiazolin-3-one, benzisothiazolin-3-one, or mixtures thereof.
The polymers of the invention are preferably finely divided and in particular have an average particle size (measured by the method of laser correlation spectroscopy) of  less than 1 xcexcm, preferably 50-800 nm, in particular 50-500 nm, and, with particular preference, 50-400 nm.
The polymers preferably contain 50 or more mol-% of acrylonitrile units. Examples of suitable comonomers are nonionic and/or ionic-ethylenically unsaturated comonomers.
Preferred nonionic comonomers are alkyl esters of acrylic and methacrylic acid, preferably C1-C8 alkyl esters, and also ethoxylated and propoxylated esters of acrylic and methacrylic acid, styrene and xcex1-methylstyrene and their derivatives, vinyl carboxylates, preferably vinyl acetate, conjugated dienes such as, for example, butadiene or isoprene, methacrylonitrile, vinyl chloride and vinylidene chloride, N-alkyl and N-aryl-substituted maleimides, and also hydrophilicizing comonomers such as acrylamides and methacrylamides, acrylic and methacrylic acid, N-vinylamides, such as, for example, 1-vinyl-2-pyrrolidone.
Examples of suitable ionic comonomers are vinylsulfonic acid and its salts, methallylsulfonic acid and also allylsulfonic acid and salts thereof, styrenesulfonic acid and its salts, (2-acrylamido-2-methyl)-propanesulfonic acid, its esters and its salts, (3-sulfopropyl)-acrylic acid and its salts, (3-sulfopropyl)-acrylic esters, and also the alkyl esters and oxyalkyl esters of said vinylic sulfonic acids.
Particularly preferred comonomers are alkyl acrylates, styrene, and butadiene/styrene mixtures. Preferably, however, it is also possible to operate with acrylonitrile as sole monomer.
In one preferred embodiment the whitened polymers of the invention are obtainable by emulsion polymerization in the presence of graftable substrates. These graftable substrates are preferably polymeric emulsifiers containing sulfonate groups. Particularly suitable are polymers of types I and II, which preferably have average molecular weights of Mw greater than 5000 g/mol. Likewise preferred are poly- and oligourethanes of type III, which preferably have an average molecular weight of  greater than 1000 g/mol.
The emulsifier of type I corresponds to the formula (I) and that of type II to the formula (II) 
in which
M denotes a cation-forming radical, preferably H, Na, K, NH4 or R3NH3,
n denotes 1 or 2,
R denotes a hydrocarbon chain having 1 to 10 carbon atoms or an aromatic radical having 6 to 10 carbon atoms, or preferably xe2x80x94(CH2)2NH(CH2)2xe2x80x94 or a radical derived from C1-C4 alkane or from benzene, methylbenzene or naphthalene,
R1 denotes H or xe2x80x94CH3,
R2 denotes H, C1-C16 alkyl, preferably xe2x80x94CH3, xe2x80x94C2H5, xe2x80x94C4H9, xe2x80x94CH2xe2x80x94C(CH3)3, xe2x80x94C14H29, xe2x80x94C16H33 or phenyl,
R3 denotes a hydrocarbon radical of an amine,
x, y are chosen such that the average molecular weight Mw of the polymers (I) and (II) is from 2000 to 500,000, preferably from 5000 to 100,000, and the ratio x:y=from 1:4 to 1:1, preferably from 1:3 to 1:1.
Particularly preferred emulsifiers are those of the formulae (I) and (II), in which
1A)
R1=CH3 and
R2=xe2x80x94CH2xe2x80x94C(CH3)3 or
2A)
R1=CH3 and
R2=CH3 or
3A)
R1=H and
R2=C6H5,
where in each case
R=xe2x80x94CH2CH2xe2x80x94,
M=Na,
n=1 and
x:y=from 1:1.0 to 1:1.1.
Very particular preference is given to the emulsifier of the type 1A).
Emulsifiers of the formulae I and II are known, for example, from EP-A 590 460.
Emulsifiers of the type III are the oligourethanes known from EP-A 400 410 which are used in connection with the free-radically initiated emulsion polymerization for preparing surface sizing agents for paper.
Particular preference is given as emulsifiers of the type III to the oligourethanes specified on p. 7 of EP-A 400 410, especially those obtainable by reacting a macrodiol and an oligoisocyanate having an NCO functionality of  greater than 2.
A macrodiol is, for example, a polyester of adipic acid, hexanediol, neopentyl glycol having an average molecular weight Mw=1700 g/mol. The oligoisocyanate is, for example, a hexamethylene diisocyanate (HDI) based trimer of the isocyanate type (f=3) and higher molecular mass oligomers (f=3) derived therefrom.
At medium and high molecular weights, suitable linear and branched polyacrylonitriles and acrylonitrile copolymers exhibit intrinsic viscosities (measured in dimethylformamide at 20xc2x0 C.) of [xcex7]=from 0.5 to 10.0 dl/g, preferably of [xcex7]=from 1.0 to 5.0 dl/g. High molecular mass products exhibit heightened long-chain branching.
Preferred polymers are those modified by strongly acidic groups. Preferred strongly acidic groups are sulfonate groups, the amount of strongly acidic groups being preferably 50-500, particularly 100-500, milliequivalents/kg of polymer.
The incorporation of strongly acidic groups into the copolymer can take place, for example:
a) by using the above-described graftable emulsifiers containing sulfo groups,
b) by using comonomers containing sulfonic acid groups and/or sulfonate groups, preferably the abovementioned ionic-ethylenic comonomers, and/or
c) by the free-radical initiator components, preferably by using the initiator system: persulfate/sulfite.
The amount of graftable substrates, especially of the abovementioned emulsifiers, which is preferably used for the emulsion polymerization is 2-20% by weight, preferably 2-10% by weight, based on the total amount of monomers to be polymerized.
The polymers obtained in the presence of graftable substrates, especially their dispersions, include not only homo- and/or copolymers of the acrylonitriles used for the polymerization and/or mixtures thereof with other monomers, preferably including fractions of graft polymer of the monomers used onto the polymeric emuslifiers, but also fractions of the ungrafted, graftable substrates, especially of the polymeric emulsifiers.
Based on the amount of emulsifier used, preferably 50 or more mol-% is grafted. Depending on the chosen polymerization conditions such as temperature, initiator system and nature of the comonomers, the grafting yield when using the polymeric emulsifiers specified as preferred is from 50 to 80 mol-%.
Particularly suitable optical brighteners are those commonly used in the textiles sector for whitening polyacrylonitrile. Particular preference is given to neutral brighteners, but especially cationic brighteners. Particularly suitable are optical brighteners which on the corresponding fiber material in the textiles sector have lightfastnesses of at least 4 as determined in accordance with the xenon test in accordance with the method by way of the guidelines determining colorfastness properties in accordance with DIN 54 004. Particularly advantageous for this application are all brighteners having lightfastnesses of 5-7.
Brigheners used with preference for the polymer dispersions are compounds of the formula: 
in which
R4 denotes ethyl or phenyl, and
Anxcex8 represents an anion, or 
xe2x80x83in which
X represents CH or N,
Anxcex8 represents an anion,
R5 represents CH3 or CH2xe2x80x94C6H5, and
R6 represents H or SO2xe2x80x94CH3, or 
xe2x80x83in which
R7 represents NH2, CH3, C2H4xe2x80x94OH, NHxe2x80x94C3H6xe2x80x94N(CH3)2, CH2xe2x80x94CH(CH3)xe2x80x94N(CH3)2, C2H4xe2x80x94CONHxe2x80x94C2H4xe2x80x94N(CH3)2, C2H4xe2x80x94Oxe2x80x94CH(CH3)xe2x80x94CH2xe2x80x94N(CH3)2, NHxe2x80x94C3H6xe2x80x94N(CH3)3+Anxcex8, C2H4xe2x80x94N(CH3)3+Anxcex8, C2H4xe2x80x94CONHxe2x80x94C2H4xe2x80x94N(CH3)3+Anxcex8, CH2xe2x80x94CH(CH3)xe2x80x94N(CH3)3+Anxcex8, C2H4xe2x80x94Oxe2x80x94CH(CH3)CH2xe2x80x94N(CH3)3+Anxcex8,
where Anxcex8 represents an anion, preferably the anion of a mineral acid or xe2x80x94O3SOCH3 or 
xe2x80x83in which
R8 denotes H or CH3, or 
xe2x80x83in which
R9 represents CH3, CH2C(CH3)2xe2x80x94CH2xe2x80x94N(CH3)3+Anxcex8 or 
where Anxcex8 denotes an anion, preferably the anion of a mineral acid or xe2x80x94O3SOCH3.
Preferred polyacrylonitriles or acrylonitrile copolymers of the invention are whitened with optical brighteners of the formula (1), (2) or (3), especially (1) or (2).
The emulsion polymerization generally takes place such that acrylonitrile and/or its mixtures with the comonomers and, if desired, graftable substrates in the presence of emulsifiers are polymerized in aqueous emulsion, using free-radical initiators, in a manner known per se.
Suitable initiators are persulfates, hydrogen peroxide and various other organic peroxides which decompose below 100xc2x0 C., and also the redox systems which respond in the temperature range from 20 to 100xc2x0 C., such as, for example, H2O2/Rongalit, H2O2/thioglycol and t-butyl hydroperoxide/ascorbic acid. Particularly advantageous is the system persulfite/SO2, which is active at temperatures of 40-70xc2x0 C. and which, by virtue of the incorporation of sulfonate and sulfate end groups, increases the level of acidic groups present.
Particularly preferred emulsifiers are the above-described polymeric emulsifiers, containing sulfonic acid groups, of the formulae (I) to (III).
To conduct the emulsion polymerization it is preferred to include all or some of an aqueous solution of the emulsifier, preferably of the polymeric emulsifier described above, in the initial charge and then to add the monomers and the initiator gradually, for example over 2 to 6 hours, preferably separately. In the case where some of the emulsifier is included in the initial charge, the remainder can then be metered in gradually, for example over 2 to 6 hours, together with the monomers and the initiator, preferably in separate streams. Alternatively, it is possible to add the emulsifier, initiator and the monomers separately and continuously throughout the polymerization period and to include only water in the initial charge.
It is also possible to operate batchwise, in which case, for example, emulsifier, monomer mixture and initiator are introduced together and this charge is then heated, for polymerization, to the desired polymerization temperature.
Depending on the decomposition characteristics of the initiators used, the emulsion polymerization is conducted at temperatures from 20 to 100xc2x0 C., preferably in the range from 40 to 80xc2x0 C.
The product of the polymerization reaction comprises generally finely divided aqueous polymer emulsions which are free from precipitated coagulum.
Thus with the process of emulsion polymerization it is possible, for example, to produce the whitened polymers of the invention as aqueous dispersions in the case of acrylonitrile homopolymers, preferably with solids contents of up to 45% by weight, and, in the case of acrylonitrile copolymers, preferably with solids contents of up to 65% by weight.
The resulting dispersions are extremely stable to additions of electrolyte, to temperature effects, and to the action of shear forces.
The optical brightener can be applied to the polymer particles, for example, in accordance with the methods described in DE-A-195 00 195. The process of the invention has the particular feature that a suitable optical brightener is added before, during or after the emulsion polymerization.
In one particularly preferred embodiment the brightener, preferably a water-soluble cationic brightener, alone or in combination with further optically brightening compounds, is mixed into the aqueous dispersions of the polymers containing strongly acidic groups, and is then caused to attach to the polymer particles at elevated temperature. If desired, further auxiliaries are used.
The attachment of the optical brightener preferably takes place above the hydrothermal softening temperature (TgH2O) of the polymer, preferably at 25-150xc2x0 C., in particular at 50-100xc2x0 C. This takes place largely with retention of the original particle size distribution of the emulsion polymers.
It is advantageous to cause the optical brighteners to act on the dispersion comprising emulsion polymers, which still contains residual monomers. Subsequently, the monomers are separated off, preferably by distillation. It has been found that, in the case of this preferred attachment variant, the takeup capacity on the one hand and the attachment rate of the brightener on the other hand are increased.
The amount of the optical brighteners used, based on the polymer, is guided by the desired brightener effect and is preferably 0.1-2% by weight of brightener, based on the polymer solids fraction in the dispersion.
The invention additionally provides aqueous dispersions containing more than 30% by weight, especially 31-65% by weight and, with particular preference, 40-65% by weight, based on the dispersion, of a whitened polyacrylonitrile and/or acrylonitrile copolymer. This aqueous dispersion of the invention contains preferably 1-15% by weight, especially 2-10% by weight and, with particular preference, 5-10% by weight of at least one surface-active substance, based on whitened polymer. The preferred embodiments of these dispersions in terms of polymer, surface-active substances and possible further additives correspond to those for the abovementioned dispersions. The polymer is preferably obtained by the method of emulsion polymerization, in particular in accordance with the process described above.
The aqueous polymer dispersions of the invention may include as further additives, like additives already described above to the whitened, finely divided polymer dispersions used in accordance with the invention, in the stated amounts, examples being surface-active substances, preservatives, etc.
The invention additionally provides acrylonitrile homo- and/or copolymers whitened with optical brighteners, said polymers having been emulsion-polymerized in the presence of at least one polymeric emulsifier of the formulae I to III.
The preferred embodiments correspond to those indicated above.
The whitened polymers of the invention, especially in the form of their aqueous dispersions, can be used without further treatment or purification or can be admixed with pigments and/or further auxiliaries.
The invention additionally provides aqueous coating slips having a solids content of 25-80% by weight, especially 50-70% by weight, and containing:
The polymer of the invention or the dispersion of the invention comprising a whitened polymer is preferably employed in the coating slips of the invention.
The coating slips of the invention may also include wet strength agents, dispersants and, if desired, further additives.
Suitable inorganic pigments are preferably clay, kaolin, barium sulfate, satin white, silica, alumina, aluminum silicate, titanium dioxide, talc, chalk and mixtures thereof.
These pigments are preferably used in the form of an aqueous slurry.
Examples of suitable binders are those based on polymeric binding agents, such as, for example, butadiene-styrene, styrene-butyl acrylate, acrylonitrile-butadiene-styrene, styrene-butadiene-alkyl acrylate, alkyl acrylate, acrylonitrile-alkyl acrylate, acrylonitrile-styrene, ethylene-vinyl chloride, and ethylene-vinyl acetate copolymers and also the homopolymers polyethylene, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, and polyaddition compounds such as polyurethanes.
Also outstandingly suitable, however, are natural binding agents such as, for example, starch, degraded starch, alginates and proteins, such as gelatin, casein and soy protein, for example. Sodium carboxymethylcellulose, polyvinyl alcohol, and synthetic, acrylate-based thickeners are also suitable. Examples of suitable dispersants are sodium polyacrylates, sodium polyphosphate and sodium citrate.
The coating slips per se into which the polymers of the invention, or the dispersions of the invention comprising whitened polymers, can be incorporated are known and are described, for example, in J. P. Casey xe2x80x9cPulp and Paperxe2x80x9d, Chemistry and Chemical Technology, 2nd Ed. Vol. III, pp. 1648-1649 and McGraw-Hill xe2x80x9cPulp and Paper Manufacturexe2x80x9d, 2nd and 5th Ed. Vol. II, p. 497.
The invention additionally provides for the use of such coating slips to coat substrates.
Examples that may be mentioned of substrates suitable for coating are:
Paper, such as coated paper, photographic paper, etc., paperboard, wood, polymer films, textiles and also nonwoven materials and building materials such as stone, plaster, etc. The abovementioned coating slips are preferably used to coat coated papers, photographic papers, and paperboard.
With the coating slip of the invention it is possible to coat paper and paperboard with the customary coating apparatus: for example, with an air knife, a coating knife, a brush, a roller, a knife coater, a rod or any other coating apparatus common in the paper industry.
The coating slips of the invention can be used in particular for coating paper, especially for chromoprints, art prints/picture prints, offset prints, gravure prints, and paperboard.
The aqueous coating slips used in accordance with the invention can comprise additional dispersants and/or emulsifiers and also customary anionic or cationic and/or nonionic surface-active substances, as are described, for example, in DE-A-2 334 769, pp. 8-10 (corresponds to GB-A-1 417 071). Preference is given in this context to anionic and/or nonionic surfactants.
The aqueous coating slips of the invention can be used, for example, as topcoats. A particular advantage of this use in accordance with the invention lies in the very high gloss and extraordinary smoothness of the coated papers obtained in this case.
The invention additionally provides the substrates, especially papers, coated with the coating slips of the invention.